1. Field of the Invention
The present invention relates to a process for producing an epitaxial silicon wafer by growing an epitaxial layer having a low boron concentration on a silicon wafer having a high boron concentration by an epitaxial growth method, and a silicon wafer produced by the process.
2. Prior Art
When an epitaxial layer in which boron was doped at low concentration was grown by an epitaxial growth method on the upper surface layer portion of a silicon wafer with low resistivity (0.003 Ω·cm or less) in which boron was doped at high concentration, misfit dislocations have been conventionally generated caused by the difference between the lattice constant of the silicon wafer and the lattice constant of the epitaxial layer. Since the misfit dislocations are moved on the shallow surface layer portion of the surface of the epitaxial layer during epitaxial growth, dislocations exist in an active zone at which semiconductor devices are prepared. Thus, when dislocations being crystal defects exist in the epitaxial layer, it causes malfunction of semiconductor devices and there is a fear of lowering their yield.
In order to solve the problem, there is disclosed a process for forming an epitaxial structure in which an epitaxial layer is deposited on a silicon substrate to which boron and germanium were doped (for example, refer to the patent literature 1: Japanese Patent Laid-Open Publication No. Sho 61-141700 (Claims 1 to 5, the third line to the eleventh line of an left upper column in page 3 of the specification and the 18th line of the right upper column to the second line of the left lower column in page 3 of the specification)). In the process for forming an epitaxial structure, when the concentration of boron doped on the silicon substrate is referred to as CB and the concentration of germanium doped on the silicon substrate is referred to as CGe, boron and germanium are doped on the silicon substrate so as to satisfy the relational formula of CB=6 CGe. Further, the thickness of the epitaxial layer is set as 5 μm and boron is doped by 0.02% by atom or more (resistivity: 0.015 Ω·cm or less).
In the epitaxial structure thus formed, the greater part of stress and corresponding strain which were caused by lattice incompatibility in an epitaxial wafer which was obtained by depositing an epitaxial layer to which a low concentration of boron was doped on a silicon substrate to which a high concentration of boron was doped are released by doping germanium in the crystal of the silicon substrate. Thereby, the epitaxial wafer does not substantially contain stress and strain is not generated irrespective of the thickness of the epitaxial layer. In other word, the degree of bending of the epitaxial wafer is made irrespective of the thickness of the epitaxial layer by the above-mentioned germanium.
Further, in order to solve the problem, there is also disclosed an epitaxial silicon single crystal wafer in which germanium was doped in the vicinity of the surface of a silicon wafer in which boron was doped in high concentration (for example, refer to the patent literature 2: Japanese Patent Laid-Open Publication No.2003-209059 (Claims 1 to 4 and paragraph [0010])). In the epitaxial silicon single crystal wafer, the concentration of boron in the wafer is 7×1018 atoms/cm3 or more and germanium is doped at a concentration of 1×1020 to 5×1020 atoms/cm3 from the surface of the wafer to a depth of 20 nm or less.
Since the radius of covalent bond of boron is small in comparison with the radius of covalent bond of silicon in the epitaxial silicon single crystal wafer thus composed, the strain which is caused by boron is compensated by supplying tetra-valent germanium which has the larger radius of covalent bond than that of silicon and an atomic valency equal to silicon.
However, in the above-mentioned conventional process for forming an epitaxial structure which was shown in the patent literature 1, when a silicon single crystal is pulled up by the Czochralski method, the segregation coefficient of boron is different from the segregation coefficient of germanium; therefore it is difficult the concentration of boron CB and the concentration of germanium CGe satisfy always the relational formula of CB=6 CGe over the full length of the silicon single crystal which was pulled up.
Further, in the above-mentioned conventional process for forming an epitaxial structure which was shown in the patent literature 1, the doping amount of germanium which is required for doping boron in high concentration is extremely much in order to lower the resistivity of a silicon substrate. For example, when the concentration of boron is 3.6×1019 atoms/cm3 or (resistivity: 0.003 Ω·cm), the weight of germanium required is numerously 1.5 kg when the weight of silicon melt is 35 kg and since germanium is very precious, there is a problem that the production cost of the silicon substrate is increased.
Further, according to the study of the present inventors, when boron is doped in high concentration and germanium is doped in high concentration, there are problems that the probability of dislocating a silicon single crystal at growth of the silicon single crystal is enhanced and the yield of an ingot at production of the silicon single crystal is lowered.
Further, in the above-mentioned epitaxial silicon single crystal wafer which was shown in the patent literature 2, since its process is a single wafer processing that germanium is doped by every wafer, the production time of a wafer is elongated and there was a problem that its production cost was greatly pushed up.
It is the object of the present invention to provide a process for producing an epitaxial silicon wafer which can reduce the doping amount of germanium, can suppress the generation of misfit dislocations and can further shorten the production time of the wafer, and a silicon wafer produced by the process.